The heart muscles of animals periodically contract to force blood through the arteries of the animal. As a result, irregularly-shaped pressure pulses exist in these arteries and cause them to flex or oscillate. The base line pressure for these pulses is known as the diastolic pressure and the peak pressure for these pulses is known as the systolic pressure A further pressure value, known as the "mean arterial pressure" (MAP), represents a time-weighted average of the pulse pressure. In the past, various techniques and devices have been used for measuring one or more of these blood pressure values. The most common method involves applying a pressure cuff about the upper arm of a test subject and inflating it so as to stop the flow of blood in the brachial artery. The pressure is then slowly relieved while a stethoscope is used on the distal portion of the artery to listen for pulsating sounds, known as Korotkoff sounds, that accompany the reestablishment of blood flow in the artery. As the pressure in the cuff is reduced further, the Korotkoff sounds eventually disappear. The cuff pressure at which the Korotkoff sounds first appear during deflation of the cuff is a measure of the systolic pressure and the pressure at which these sounds disappear is a measure of the diastolic pressure. This method of blood pressure detection is generally known as the ausculatory method.
Various devices are well known in the prior art for automatically performing blood pressure measurements by the ausculatory method. These devices employ a pump to automatically inflate a pressure cuff and a microphone to convert the Korotkoff sounds into electrical signals which are easily detected by various types of circuits. Other techniques have also been used to detect blood pressure from outside the subject's body, e.g., via Doppler shifts in ultrasonic waves reflected by the artery wall. In addition, there are intrusive devices that are inserted directly into the blood vessels for measurement of the pressure. However, the most commonly used method for measuring blood pressure, other than the ausculatory method, is the oscillometric method.
The oscillometric technique is based on the fact that the pumping of blood through the arteries by the heart causes the arteries to flex. Even in the area adjacent to or within a pressure cuff applied to the arm of a test subject, these pressure variations exist. In fact they will pass from the artery through the arm of the test subject and into the pressure cuff itself. While these pressure variations are small compared to the typical pressure applied by the cuff, they are nevertheless detectable by a transducer located to measure the pressure within the cuff. It has been found that these pulses have a peak-to-peak amplitude which is minimal for applied cuff pressures above the systolic pressure and below the diastolic pressure. The amplitude of these pulses, however, rises to a maximum value at a cuff pressure equivalent to the mean arterial pressure. It has further been found that the pulse amplitudes for cuff pressures equivalent to the systolic and diastolic pressures have a fixed relationship to the pulse amplitude at the mean arterial pressure. Thus the oscillometric method is based on measurements of detected pulse amplitudes at various cuff pressures.
Automated blood pressure measuring devices operating according to the oscillometric method have been proposed in which the peak-to-peak amplitude of the pressure pulsations are detected at various applied cuff pressures. The amplitudes of these pulses, as well as the applied cuff pressure, are stored together as the device automatically changes the cuff pressure over the range of interest. Then the peak-to-peak amplitudes are evaluated to find the maximum and its related cuff pressure, which is designated the mean arterial pressure (MAP). The cuff pressure below MAP which produces a peak-to-peak pulse amplitude having a certain fixed relationship to the peak-to-peak value at MAP, is designated as the diastolic pressure. Likewise, the equivalent cuff pressure above MAP which results in pulsations having an amplitude with a certain fixed relationship to that at MAP, is designated as the systolic pressure. In modifications of this basic oscillometric technique, derivatives of the pulse wave forms are taken and maximums of the derivatives are used as indications of the occurrence of the systolic and diastolic pressures. The relationships of systolic and diastolic pressures, respectively, to MAP, are empirically derived ratios which assume varying levels depending on the preferences of those of ordinary skill in the art. Generally, these pressures are calculated in the range of 30 to 70% of MAP.
In many situations, the speed with which blood pressure readings are taken is not critical, although a delay in making a measurement may represent an inconvenience to the test subject. However, in certain situations, for example, during surgery or during the emergency treatment of patients who have suffered severe trauma, it is often necessary to obtain the blood pressure reading quickly and to obtain repeated readings over a period of time. With the automatic oscillometric blood pressure devices known from the prior art, the cuff pressure is either increased in increments until the desired readings are obtained (an incrementing device) or it is rapidly brought to a high pressure, which is thought to be above systolic pressure, and then decreased in increments (decrementing device). Since normal blood pressure lies in the range between about 70 and 120 millimeters of mercury, one technique for improving the speed with which the blood pressure readings can be taken is to start with a reasonably high value of initial pressure, for example, 50 to 70 millimeters of mercury, when an incrementing device is used. Similarly a decrementing device could start with a relatively low pressure with respect to systolic, for example, 120 to 140 millimeters of mercury. Thus, the portions of the pressure range where it is unlikely that useful information will be obtained are skipped over and the measurement speed is increased.
The difficulty with skipping part of the pressure range to speed up the process is that a person in shock may have such a low blood pressure that the automated operation may completely miss significant information, e.g. the diastolic pressure. Likewise, if a person is suffering from arterial disease, his blood pressure may be extremely high and a decrementing device with too low an initial cuff pressure may not detect the systolic pressure. Besides the people with heart disease or who are in shock, certain people naturally have blood pressures outside the normal range. Typical of this is the blood pressure of long distance runners which tends to be much lower than that of the general population. With such people, a blood pressure reading may not be possible with a device in which part of the pressure range is skipped in order to obtain faster readings.
Of course, a skilled operator, upon failing to get a reading, can reset the machine to search for blood pressure in a lower or higher range as indicated. However, in critical situations in which speed is of the essence, this may not be accomplished easily. In particular, the operator may assume that the failure to read a blood pressure is due to a failure of the machine, as opposed to an abnormally high or low pressure in the test subject. Thus, valuable time may be lost in trying to check out the equipment. Further during emergency treatment or surgery, it may not be convenient for the operator, for example, a paramedic or an anesthesiologist, to reprogram the machine since he may be engaged in other critical life saving operations.